Screen printing techniques have long been used to fabricate printed circuit boards and other substrates for the electronics industry, wherein conductive inks are applied to the substrate through a patterned screen and fired to remove the organics, leaving a patterned layer of the conductor, e.g. copper, on the substrate.
As the patterns became smaller and more intricate, screen printing was supplemented by photoresist technology. The conductive material was put down as a thin layer, a photoresist layer applied thereover, and then exposed and developed to form a patterned layer. The uncoated conductive layer was etched away, and the remaining photoresist dissolved away or fired, leaving a patterned conductive layer on the substrate.
This basic technique has also been applied to other layers, such as dielectric layers made from devitrifying glasses which are mixed with an organic vehicle to form a paste. The paste is applied to the entire substrate, which substrate can be covered by a patterned conductive layer, and dried. In order to gain access to the underlying conductor pattern, via holes are formed in the dielectric layer which is to be filled with a conductor such as copper. A photoresist layer is applied over the dried dielectric layer, the resist is exposed using a photo tool to define the via holes, the resist developed to remove unexposed photoresist, and the dielectric washed away to form via apertures. The substrate is then fired in an appropriate atmosphere, e.g. nitrogen for copper, air for inert conductors such as silver and gold, whereupon the organic materials, the paste organic vehicle and remaining photoresist, burn away, and the glass dielectric sinters onto the substrate. The via holes are then filled with copper using screen printing techniques.
The above technique is eminently suitable to form via holes between about 100 and 250 micrometers in diameter. However, with the increased miniaturization required for the electronics and computer industries, it is desired to make smaller via holes between about 25 and 125 micrometers in diameter.
Conventional screen printing or photoresist techniques have grave problems when very small via holes through comparatively thick layers of, inter alia, dielectric, are to be made.
The dielectric paste in the small vias is very difficult to remove, especially when the dielectric layer is quite thick. The dielectric cannot be completely removed from the side walls, and in some cases cannot be removed down to the bottom of the via hole. In addition, it is difficult to fill small holes with copper paste using screen printing techniques.
Further, the photoresist is absorbed by the dielectric ink to some extent during the drying step. Thus when the photoresist is subsequently exposed to ultraviolet light, the dielectric is rendered gummy which further interferes with removal of the dielectric layer. This exacerbates the difficulty of removal of portions of the dielectric paste.
Another problem encountered when making very small patterns and via connections is that all of the various unfired or fired layers of conductor and dielectric inks, as well as specialty inks such as dielectric overglaze inks and the like, undergo some absorption of the conductive copper into underlying and overlying dielectric layers, and some absorption of photoresist into both the copper layers and the dielectric layers. While these problems are present to some extent in the manufacture of all printed circuit boards, they are more troublesome when very small patterns and via holes are to be made.